Elucidation of the fundamental mechanisms by which the carotid body chemoreception of O2 and CO2-H+ occur is our broad and long-range goal. The consensus model is that the innervated glomus cell is the presynaptic whereas the nerve endings in appositions are the postsynaptic elements of the chemoreceptor unit. A direct consequences of this model is that the sensory discharge is driven by the events occurring in the innervated glomus cells. Accordingly the sensory responses provide a reliable window to the functions of the intact glomus cells in their natural environment in the intact carotid body, and dopamine release (a putative neurotransmitter), a direct response of the glomus cells (being the only source of dopamine). A link-between the two responses would indicate a cause and an effect relationship. The hypotheses are that CO2 acts intracellularly by catalyzed production of H+ which evokes Ca2+ entry through the functions of the ion-exchangers and that the hypoxic effects are not mediated by the same mechanism as H+. The hypotheses and model will be tested by means of the following specific aims: (I) Define the role of CO2-HCO3, intra- and extracellular pH and intracellular carbonic anhydrase. (II) study the role of ion-exchangers and ion-channels in the responses to CO2-H+ and hypoxia, and the dependence of carbonic anhydrase effects on the ion-exchanger function. (III) Role of Ca2+ entry vs. intracellular calcium stores in sensory and dopamine responses. (IV) Dopamine release at the hypoxic and nonhypoxic stimulus thresholds (PaO2- PaCO2) for chemosensory discharge, and dependence of dopamine release due to hypoxia on oxidative metabolism. To achieve these objectives we will extensively use our purposefully developed in vitro carotid body preparation, perfused and superfused with cell-free solutions, for the measurement of carotid chemosensory discharge, catecholamine release and intraglomeral pH. In addition, we will use in vitro incubation of carotid body for catecholamine release and turn-over rate, and in vivo carotid body preparation for dopamine release at zero chemosensory discharge at low and high arterial Po2 (Pao2-Paco2 stimulus thresholds). Understanding the mechanisms of initiation and control of carotid body functions which strongly influence respiratory and cardiovascular reflexes in health and disease is crucial.